Organic light-emitting devices (OLEDs) are finding applications as next-generation flat-panel displays (FPDs), liquid crystal displays (LCDs), and plasma display panels (PDPs). This has been driven by their favorable properties including lightweight, fast video response and low power consumption. To this end, organometallic compounds exhibiting electroluminescence are particularly attractive for electrophosphorescent applications, since both the ligand structure and the central metal atom can be varied to modify the properties of the device using these compounds.
An organic light-emitting device (OLED) is an energy conversion device which emits light when current is applied. A multilayer OLED is generally comprised of hole and electron injection layers, hole- and electron-transporting layers, an emissive layer, metal oxide layer and metal electrodes. The use of organic small molecules and polymers in the emissive layer has attracted much attention due to their potential applications in full-color large-area flat-panel displays. Tang and VanSlyke first disclosed that organic small molecules can be prepared as thin-films by vacuum deposition to form multilayer organic light-emitting devices (OLEDs) (see Tang et al., Appl. Phys. Lett. 51:913, (1987)).
Investigations on organic small molecules have been made in order to improve the performance of OLEDs. In general, fluorescent and phosphorescent materials are employed as light emitters in the emissive layer of OLEDs. Light emission from a fluorescent compound occurs as a result of formation of singlet excitons in the emissive layer of the electroluminescent device. U.S. Pat. No. 6,310,360 disclosed that theoretically 25% singlet excitons and 75% triplet excitons are produced after recombination of holes and electrons in the emissive layer of an electroluminescent device. The singlet excitons transfer their energy to the singlet excited state while the triplet excitons transfer their energy to triplet excited state. Most of the organic small molecules exhibit fluorescence; hence, only 25% of the generated excitons are utilized resulting in the device with low external efficiency.
In contrast to fluorescent compounds, a series of effective phosphorescent iridium complexes with different color emissions has been reported jointly by Thompson et al. at the University of Southern California and Forrest et al. at Princeton University (see U.S. Pat. No. 6,515,298 B2; U.S. Patent Application Publication No.20020182441 A1; Lamansky et al., J. Am. Chem. Soc., 123:4304 (2001); and Xie et al., Adv. Mat., 13:1245 (2001)). Che et al. also demonstrated the use of organic metal complexes employing various metal centres such as platinum(II), copper(I), gold(I), and zinc(II) as OLED emitters (see U.S. Patent Application Publication No. 23205707 A1; U.S. Patent Application Publication No. 22179885 A1; Y.-Y. Lin et al., Chem. Eur. J., 9:1263 (2003); Lu et al., Chem. Commun., 206 (2002); Ma et al., New J. Chem., 263 (1999); Ma et al., Appl. Phys. Lett., 74:1361 (1999); Ho et al., Chem. Commun., 2101 (1998); and Ma et al., Chem. Commun., 2491 (1998)).
A variety of light-emitting compounds, especially red emitters, have been investigated as active emitters in a number of device structures. U.S. Pat. No. 6,048,630 disclosed OLEDs based on phosphorescent Pt(OEP) complex (H2OEP=octylethylporphyrin) which emits saturated red electroluminescence. Thompson and Forrest et al. reported a red phosphorescent material (bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N, C3)iridium(acetylacetonate) [Btp2Ir(acac)]) with high-efficiency (ηext=7.0±0.5%) (see Adachi et al., Appl. Phys. Lett., 78:1622 (2001)). In addition, europium complex employed as red emissive dopant in OLED (Eu(TTA)3phen, TTA=thenoyltrifluoroacetone; phen=1,10-phenanthroline) was also reported to show sharp red electroluminescence (see Adachi et al., J. Appl. Phys., 87:8049, (2000)).
Efforts in the development of red phosphorescent emitters with high efficiency for OLEDs are geared towards the full-color flat panel display application. Even though remarkable progress has been made, challenges such as optimization of stability and efficiency of OLEDs need to be met before commercialization. It is, therefore, particularly contemplated to develop phosphorescent materials, which exhibit electroluminescent (EL) emissions in visible light region, with high efficiencies and good stabilities.